Azeotropic distillation of hydrocarbons with tetrahydrofuran



Patented May 9, I950 AZEOTROPIC DISTILLATION F HYDRO- CARBON S WITH TETRAHYDROFURAN Morgan W. Davidson, Bartlesvllle, 0th., alslgnor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application Degembcr 6, 1848,

Serial No. 63,84

10 Claims.

I'hls invention relates to the separation of hydrocarbon mixtures into their components. In preferred embodiments the invention relates to the separation of saturated hydrocarbons, by which term I mean parafllns and/or naphthenes (sometimes referred to as cycloparafllns), from admixture with aromatic hydrocarbons. Inother embodiments the invention relates to the separation of paraflin hydrocarbons from naphthenes having similar boiling points. In certain specific aspects the invention pertains to the treatment of narrow-boiling range mixtures containing normal hexane and benzene to separate the one from the other by use of a novel entraining agent.

Numerous instances are known in which the separation between hydrocarbon types obtainable by simple fractional distillation is inadequate, because of the closeness of the boiling points of two or more individual hydrocarbons of varying types and/or because of the formation of con stant' boiling mixtures between such hydrocarbons of different types. In such cases it becomes necessary to find some other means of eflecting a final separation so that one or more individual hydrocarbons may be obtained in a sufliciently pure form for the desired use. Mixtures of saturated and aromatic hydrocarbons are often obtained from naturally occurring petroleum sources such as straight run gasolines or natural gasolines. They are also obtained from the pyrolytic or catalytic conversion of lighter or heavier hydrocarbon materials and from various synthetic' reactions which produce hydrocarbon mixtures. Except in natural gasoline and straight run gasolines, such mixtures also usually contain unsaturated hydrocarbons such as olefins, diole- :dns and acetylenes, and these unsaturated materials may be removed by acidtreatment or other methods known to the art to produce a mixture composed only of paraflins and/or naphthenes, together with aromatic hydrocarbons.

. The hydrocarbon mixtures of the type described which are of the most general interest are those which contain-the low-boiling aromatic hydrocarbons benzene and/or toluene, together with the straight chainparaflins, the various branched chain paraflins, and the naphthenic hydrocarbons with or without alkyl side chains having boiling points close to the aforesaid aromatics. Often it is desirable to recover the arcmatic hydrocarbons in question from all nonaromatic hydrocarbon material for particular uses requiring ahlgh-purity "aromatic, whereas at other times it is of most interest to remove the aromatics completely from the saturated hydrocarbons so that the latter may be employed for particular uses. Thus, the branched chain poraflins in general have relatively high octane numbers and are usable as such in gasolines for internal combustion engines. The same is true of the naphthenlc hydrocarbons. The straight chain or slightly branched chain hydrocarbons on the other hand have extremely low or moderate octane numbers, but are readily converted to high octane number materials by catalytic lsomerlzatlon processes. Some fuels are desired which are free from aromatic hydrocarbons so that removal or same from the saturated hydrocarbons is necessary. It has also been found that when a paramn such as normal hexane, for example. is subjected to isomerization in the presence of a metal halide catalyst, e. g. an aluminum chloride-hydrocarbon complex, even small quantitles of benzene or other aromatic hydrocarbons are extremely detrimental to the catalyst and must be removed.

By way of a more specific example, it may be pointed out that by fractionation of gasoline derived from Mid-Continent crude oils a fraction may be obtained by simple fractional distillation which comprises about 96 per cent normal hexane, 3 per cent benzene and minor proportions of methyl cyclopentane and i-carbon atom hydrocarbons. In some cases as much as 20 per cent methyl cyclopentane and as little as 76 per cent normal hexane alongwith three per cent benzene are obtained as overhead product. This material cannot be resolved into its components by ordinary fractionation since benzene and normal'hexane appear to form an azeotrope containing about three volume per cent benzene (Ind. Eng. Chem. 38, 264 (1946)). It is particularly desirable to remove benzene from normal hexane which is to be used as a feed stock to isomerizatlon processes in which Friedel-Crafts type catalysts are used. Benzene has the eil'ect of increas- I ing catalyst consumption and thus has an unfavorable eli'ect on the economics of the isomerlzation process. In the case where normal hexane is isomerized with an aluminum chloride sludge catalyst, relatively small quantities oi benzene have a profound en'ect on catalyst consumption, and cause a very undesirable thickening of the liquid complex catalyst. On the other hand methyl cyclopentane is not particularly undesirable in the isomerization feed stock, and limited quantities have a beneficial efl'ect by lncreasing the specificity of the isomerization' reaction so that production or lower boiling compounds is minimized Itis an object of this invention to separate hydrocarbons according to type. It is another object of the invention to effect the resolution into its components of a narrow-boiling-range mixture of hydrocarbons having a boiling point within the range of about 65 to about 125 C. and

containing at least two of the types parafhns,

naphthenes, and aromatics. A further object is to prepare pure normal hexane. Yet another ,ene from admixture with saturated hydrocarbons which are either close boiling or which contain the benzene and/or toluene as a component of a constant boiling mixture. Another object of the invention is to provide an improved azeotropic distillation process. Further objects and advantages of the invention will be apparent to one skilled in the art, from the accompanying disclosure and discussion.

I have discovered that separations of saturated hydrocarbons from low-boiling aromatic hydrocarbons such as benzene and toluene are readily effected by subjecting mixtures containing same to azeotropic distillation in the presence of tetrahydrofuran as an entraining agent. By employ ing sumcient tetrahydrofuran to form a low-boiling azeotrope with at least a portion of the satu rated hydrocarbons present in such a mixture, I

am able to recover as overhead distillate, from either a batch or cont nuous azeotropic distillation, an aromatic-free material. On removing the tetrahydrofuranfrom this distillate there is obtained a saturated hydrocarbon or mixture of saturated hydrocarbons which is free from contamination with benzene or toluene. I likewise obtain as a high-boiling kettle product a hydrocarbon material enriched in aromatic hydrocarbon content. This kettle product may or may not contain tetrahydrofuran depending on the quantity of the latter which has been used in the distillation and the extent to whichthe feed to the fractional distillation has been distilled overhead. If parafilns, naphthenes and aromatics are all present, the parafllns are concentrated in the overhead and the aromatics are concentrated in the bottoms. while the naphthenes may be concentrated either in the bottoms or the overhead as desired depending on the proportion of feed material distilled overhead. If a mixture containing only paraillns and naphthenes is treated, the former are recovered overhead as the lower boiling azeotrope.

Tetrahydrofuran is a stable liquid material having a boiling point of about 65 C. at normal atmospheric pressure, and which is readily soluble in water. I have found that it is an eiilclent entraining agent foraccomplishing the separations herein described, and that it is readily removed from the distillate and/or the kettle product by simple addition of water thereto, allowing a ready recovery of the desired hydrocarbon mixtures.

As applied to hexane-benzene mixtures of the type described hereinabove, the invention comprises the fractional distillation of such a mixture with organic solvents, and the like.

4 in the presence of added tetrahydrofuran in a column of relatively high efllclency, an overhead product being recovered comprising a binary mixture of normal hexane and tetrahydrofuran. when the latter is removed from this overhead product by some convenient method, such as water washing, a pure normal hexane productis obtained. This product is suitable for use in isomerlzation processes using Friedel-Crafts type catalysts, or in other processes in which a benmne-i'ree normal hexane product is required.

While the example cited hereinafter refers to batchwise fractional distillation, m process is.

equally applicable to continuous fractionation. In case continuous fractionation is employed, the hydrocarbon mixture 'is continuously charged to the column at some point along the column and the entralnlng agent added at the same or other points along the column. In such a continuous fractionation, normal hexane and entrainer are removed asan overhead product. The hydrocarbons removed from the kettle; principally benzene, and methyl cyclophentane, may or may not contain dissolved entrainer depending on the relative amounts of hydrocarbon and entrainer charged to the column. My process may be operated with any desired ratio of entrainer to hydrocarbon that is equal to or greater than a minimum ratio which will be determined by the amount of entrainer required to form azeotropic mixtures with the hydrocarbon distilled overhead.

In my process I use any convenient method of removing the entrainer from the overhead and/or kettle products such as water washing, extraction The tetrahydrofuran is recovered and reused in the process. Thus, sumcient water may be added to the products from the fractionator to cause a phase separation, the hydrocarbon phase is separated and washed with additional water if necessary, and the aqueous phase or phases may be distilled to recover the tetrahydrofuran from the water.

It is' preferred that before subjecting a mixture of hydrocarbons to an azeotropic distillation with tetrahydrofuran, the mixture should first be subjected to ordinary fractional distillation to prepare one or more narrow boiling-range fractions.

Such a narrow boiling-range fraction preferably has a boiling range not exceeding 5 to 10 F., and the most efllcient separation is obtained by subjecting a fraction of this nature to the azeotropic distillation.

A specific example of my process which illustrates its eflectiveness in obtaining pure normal hexane from hexane-benzene mixtures is as follows. It will be understood, of course, that the invention in its broad scope is not limited to the exact conditions and materials of this particular example.

To 150 ml. of a mixture of 3.5 per cent by volume benzene and 96.5 per cent normal hexane (containing a trace. of methylcyclopentane) and having a refractive index (No of 1.3793, was added 50 ml. of tetrahydrofuran. The resulting mixture was charged to a packed fractionating column having approximately equivalent theoretical plates, and fractionated batchwise at atmospheric pressure using a 75:1 reflux ratio. A first overhead fraction of 25 ml. was collected which upon water washing to remove the tetrahydrofuran was found to comprise 44.! volume per head temperature of about 63 C. After removing the tetrahydrofuran from the distillate by water washing, there remained 15.8 ml of hydrocarbon (465 volume per cent of the fraction) shown to be essentially pure normal hexane by its refractive index (No of 1.3750. A third overhead fraction of ml. was composed of 42.7 volume per cent hydrocarbon, which was substantially pure normal hexane as shown by its refractive index (ND of 1.3748. By continuing the distillation in this manner further amounts of essentially benzene-free normal hexane are recovered. By providing more than enough tetrahydrofuran to form low-boiling azeotropes with all of the hydrocarbon charged, much greater quantities of the normal hexane are recovered and benzene is concentrated in a final kettle product usually admixed with some residual hexane.

While the invention has been described in detail in terms of its preferred embodiments, it is to be understood that various modifications may be made by one skilled in the art, in view of the instant disclosure. Thus, when a hydrocarbon mixture is subjected to an initial fractional distillation to produce a narrow-boiling-range fraction containing paraflins, naphthenes and aromatics, which fraction is then subjected to azeotropic distillation with tetrahydrofuran, the naphthenes may be recovered in admixture with either the paraffins in overhead or the aromatics in the kettle, or as an intermediate side out in a column having a large number of theoretical plates, or the paraflins may be distilled oil from the naphthenes and aromatics in a first column and the latter then passed to a second column wherein the naphthenes are distilled off from the aromatics, or the paraflins and naphthenes may be produced as top product of a first column and passed as feed to a second column from which the naphthenes are recovered as bottoms. These and other variations which will occur to those skilled in the art are within the broad scope of the invention.

I claim:

1. A process which comprises subjecting a narrow boiling range mixture containing hydrocarbons of at least two of the series paraflins, naphthenes and aromatics and boiling within the range of 65 to 125 C. to azeotropic distillation in the presence of tetrahydrofuran, and recovering a low-boiling azeotrope of tetrahydrofuran with at least a portion of the hydrocarbon present in said mixture which is earliest in said series.

2. A process which comprises subjecting a mixture of saturated and aromatic hydrocarbons boiling within the range of 65 to 125 C. to azeotropic distillation in the presence of tetrahydrofuran.

3. Aprocess which comprises subjecting a narrow boiling range mixture of saturated and aromatic hydrocarbons boiling within the range of 65 to 125 C. to azeotropic distillation in the presence of tetrahydrofuran, and recovering a lowboiling fraction enriched in saturated hydrocarbons and a high-boiling fraction enriched in arcmatic hydrocarbons.

4. A process for purifying a saturated hydrocarbon material boiling within the range of 65 to 125 C. andcontaminated with small amounts of an aromatic hydrocarbon which comprises admixing at least suflicient tetrahydrofuran with such a material to form a low-boiling azeotrope with at least a portion of the saturated hydrocarbon content thereof, subjecting the resulting admixture to fractional distillation, recovering an overhead distillate comprising a low-boiling azeotrope of saturated hydrocarbons with tetrahydrofuran, and separating tetrahydrofuran from said overhead distillate to produce a purified saturated hydrocarbon having a diminished content of aromatic hydrocarbons.

5. A process which comprises recovering by ordinary fractional distillation from a wide boiling range mixture a normal hexane fraction containing a small amount of benzene, subjecting said fraction to fractional distillation in the presence of tetrahydrofuran to distill overhead a benzenefree azeotrope of normal hexane with tetrahydrofuran, and removing tetrahydrofuran from said azeotrope by washing with water to produce benzene-free normal hexane.

6. A process which comprises admixing tetrahydrofuran with a narrow boiling range hydrocarbon material containing a paraflinic hydrocarbon, a naphthenic hydrocarbon and an aromatic hydrocarbon selected from the group consisting of benzene and toluene, said hydrocarbons boiling within the range of 65 to 0., and subjecting the admixture of same with tetrahydrofuran to fractional distillation to recover an overhead product enriched in paraifinic hydrocarbon and a kettle product enriched in aromatic hydrocarbon.

'7. A process which comprises subjecting a narrow boiling range mixturecontaining normal hexane, methylcyclopentane and benzene to azeotropic distillation in the presence of tetrahydrofuran, and recovering as an overhead product a low-boiling azeotrope of tetrahydrofuran with normal hexane and at least a portion of sai methylcyclopentane.

8. A process which comprises subjecting a narrow boiling range mixture containing normal hexane, methylcyclopentane and benzene to azeotropic distillation in the presence of tetrahydrofuran, and recovering as an overhead product a low-boiling azeotrope of tetrahydrofuran with normal hexane and leaving at least a portion of said methylcyclopentane in a residue with benzene.

9. A process which comprises separating a paraffin hydrocarbon and a naphthene hydrocarbon from each other, said hydrocarbons boiling close together and within the range of 65 to 125 C., by azeotropic distillation with tetrahydrofuran.

10. A process which comprises separating a naphthenic hydrocarbon and an aromatic hydrocarbon from each other, said hydrocarbons boiling close together and within the range of 65 to 125 C., by azeotropic distillation with tetrahydrofuran.

MORGAN W. DAVIDSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,162,963 McKittrick June 20, 1939 2,352,534 Greenburg June 27, 1944 2,415,192 Rittenhouse Feb. 4, 1947 2,419,521 Waldron Apr. 22, 1947 2,461,993 McKinnis Feb. 15, 1949 OTHER REFERENCES Mair et al.: Bureau of Standards Journal of Research, vol. 27, pages 44-57. 

1. A PROCESS WHICH COMPRISES SUBJECTING A NARROW BOILING RANGE MIXTURE CONTAINING HYDROCARBONS OF AT LEAST TWO OF THE SERIES PARAFFINS, NAPHTHENES AND AROMATICS AND BOILING WITHIN THE RANGE OF 65 TO 125*C. TO AZEOTROPIC DISTILLATION IN THE PRESENCE OF TETRAHYDROFURAN AND RECOVERING A LOW-BOILING AZEOTROPE OF TETRAHYDROFURAN WITH AT LEAST A PORTION OF THE HYDROCARBON PRESENT IN SAID MIXTURE WHICH IS EARLIEST IN SAID SERIES. 